1. Field of the Invention
The present invention generally relates to a lighting device. More particularly, the present invention relates to a harmonized strategy used in a lighting device for eliminating acoustic resonance in a fluorescent lamp.
2. Description of the Related Art
Gas-discharging lamps have been the most prevalent lighting sources since their development in the 1930's. They possess advantageous features including high color rendering, soft-visualization, and low energy consumption, etc. Today, fluorescent lamps are still commonly used.
A general configuration of a lighting apparatus using a fluorescent lamp is depicted in FIG. 1. Referring to FIG. 1, a voltage source V.sub.D is fed to the fluorescent lamp 10 through the ballast 20. The capacitor C.sub.S disposed across one terminal of the electrode 12a and one terminal of the electrode 12b serves as a starting capacitor. The inductor L.sub.R connected between the other terminal of the electrode 12a and the ballast 20 serves as a resonant inductor. The ballast 20 is comprised by an inverter for providing high frequency (about 20k Hz.about.65k Hz) driving voltage. Therefore, FIG. 1 shows a general configuration of a series-resonant inverter (SRI) for electronic ballast. Before igniting the fluorescent lamp 10, the inside of the fluorescent lamp 10 is not in a condition state and thus the resonant inductor L.sub.R, the filament resistance, and the starting capacitor C.sub.S make up a series-resonant circuit. After igniting the fluorescent lamp 10, the inside of the fluorescent lamp 10 is in a conduction state, and equivalent to resistors shunted with the capacitor C.sub.S.
In the last decade, versatile fluorescent lamps have been developed for improving the quality of lighting environments. Nowadays, it is the trend to develop multi-functional lamp systems with dimming control, while maintaining high power quality, to achieve a more comfortable lighting environment for humans. High power factor correction for raising the power quality is available in lamp design. However, when utilizing low-level dimming control, a low frequency snake-like circulation due to acoustic resonance in the lamp inevitably disturbs the dimming performance. This phenomenon is depicted in FIG. 2. Inside the fluorescent lamp 10, the hot electron beam 16 is in a state similar to a standing wave; therefore, the area 18 inside the fluorescent lamp 10 presents darker illumination due to lack of electron stimulation. Moreover, the current in the fluorescent lamp (lamp current) is disturbed and modulated due to the effect of acoustic resonance, resulting in the phenomenon of a standing wave. This phenomenon (acoustic resonance) leads to the igniting of arc voltage in the lamp, which may be unstable, flicker, deform, deflect, and even disappear. Besides, it may disturb the operation of the lamp and raise the lamp temperature.
Three kinds of techniques have been tried to solve the mentioned resonance in the lamp. Zollweg tried to change the gas ingredients or the lamp geometry to eliminate acoustic resonance, as described in the paper "Arc instability in mercury and metal halide arc lamp," J. of the illuminating Eng. Society, pp. 90-94, January 1979. In fact, the method disclosed in Zollweg is difficult to realize practically. Eaehnrich presented a frequency modulation technique to modulate the lamp current to be out of the resonant band, as described in the paper "Electronic ballast for metal halide lamp," J. of the illuminating Eng. Society, pp. 131-141, Summer, 1988. However, the lamp power is unstable and may be changed. Recently, Laskai disclosed an FM PWM strategy to spread the lamp power in different bands and reduce the amplitudes of the spread harmonics in order to eliminate the resonance occurring in the gas-discharging lamp in the paper "A unity power factor electronic ballast for metal halide lamps," Proc. IEEE APEC'94, pp.31-37, 1994. However, it is still useless for spreading the energy of the lower-order harmonics and will result in deterioration of the EMI in the ballast. Therefore, the above methods can not eliminate the acoustic resonance effect when the fluorescent lamp is in a low-level dimming condition.